The overall objectives of this research are an analysis of the factors controlling both the cessation of cell growth under conditions of high cell density and the resumption of growth when conditions of low cell density are restored. A sensitive parameter of cellular growth is ribosomal RNA (rRNA) synthesis and particular emphasis is therefore placed on ascertaining those factors that may be closely related to both the onset and the decline of rRNA synthesis during the growth cycle. The specific areas proposed for analysis are: 1) the cellular levels of c-AMP and guanosine tetraphosphate, 2) the cellular RNA polymerases and rRNA methylases, 3) the synthesis of ribosomal proteins. We have also observed a rapid initial increase in the capacity for protein synthesis that occurs upon dilution to low density. This process is undoubtedly a biochemical prerequisite for the subsequent onset of rRNA synthesis. At the least it is the earliest phenomenon known to occur as a consequence of dilution from the stationary phase. We propose to analyze its mechanism in terms of the levels of initiator tRNA and of protein chain initiating activity.